Reactor core in LMFBR type reactor

As the thickness of blankets in LMFBR type reactors is increased, although the breeding ratio is increased, the size of the reactor core is enlarged to increase the construction cost. Further, re-processing for the fissile materials becomes excessively large to increase the production cost for the blanket fuel assemblies. On the other hand, if the thickness of the blanket is reduced, although the size of the reactor core can be made smaller, the breeding ratio is decreased and the neutron loss due to wetting is increased even in a case where the blanket is surrounded with reflectors. In view of the above, in the present invention, fertile materials and neutron moderators are mixed together in a blanket region. The neutron moderators moderate high energy neutrons leaked from the reactor core to lower the energy and neutrons are captured more in U-238 having large neutron cross section to low energy neutrons. The conversion ratio from U-138 to Pu-239 is increased to improve the beeding ratio. Thus, it is possible to reduce the size of the structure and decrease the cost. (K.M.)